Plasma arc heating apparatus

ABSTRACT

Plasma arc gas heating apparatus comprising in combination: a helically wound high frequency induction coil, means coaxially mounting said coil in a tubular enclosure open at one end for exit of heated gas and closed at the opposite end, a pair of spaced electrodes mounted in said closed end of said tubular enclosure, means for injecting a flow of gas therein between said electrodes and thence through and over said coil, additional means for injecting gas into said tubular enclosure at a point between said electrodes and said coil, and electrical connections to said electrodes for impressing a gas ionizing voltage therebetween.

219-4214; OR 395869905 SR [72] inventor RodolpheBignell Varennes, Quebec, Canada [2 I] Appl. No. 829,923 [221 Filed June 3, 1969 [45] Patented June 22, 1971 (73] Assignee Canadian Titanium Pigments Limited Montreal, Quebec, Canada [54] PLASMA ARC HEATING APPARATUS [56] References Cited I UNITED STATES PATENTS 3,324,334 6/1967 Reed 315/111 X Primary ExaminerRaymond F. Hossfeld Attorney-Ward, McElhannon, Brooks and Fitzpatrick ABSTRACT: Plasma arc gas heating apparatus comprising in combination: a helically wound high frequency induction coil, means coaxially mounting said coil in a tubular enclosure open at one end for exit of heated gas and closed at the opposite end, a pair of spaced electrodes mounted in said closed end of said tubular enclosure, means for injecting a flow of gas therein between said electrodes and thence through and over said coil, additional means for injecting gas into said tubular enclosure at a point between said electrodes and said coil, and electrical connections to said electrodes for impressing a gas ionizing voltage therebetween,

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QTTO FEN EWE.

PLASMA ARC HEATING APPARATUS This invention pertains to improvements in high frequency, induction type plasma arc gas heaters.

A conventional type of plasma arc gas heater, consists essentially of a helically wound coil of constant pitch or spacing between turns axially thereof and through the core of which a stream of gas is caused to flow under pressure. A source of high frequency oscillations is impressed across the coil, and gas ionization is initiated by striking a small electric arc within the coil core, as for example, from a spark plug assembly. The high frequency magnetic field established by the coil dissociates a portion of the gas molecules into ions and electrons allowing them to recombine at the inductor output where energy is given out in the form of heat. There finally results the buildup of a steady state plasma arc condition, the arc of which is concentrated in a relatively thin tubular annulus within the coil. Electrical power is transferred from the coil to the plasma arc annulus by production of eddy currents therein, the action being similar to that of power transfer from a high frequency coil into a metal rod disposed within the coil. In either case, the power transfer is to a relatively thin tubular annulus owing to the high frequency skin effect."

A plasma arc gas heater of this conventional type has the disadvantage that the electrical power transfer from the high frequency coil into the plasma arc is relatively inefiicient, in that not more than about 50 percent of the power is transferred into the plasma arc. The present invention provides a novel form of plasma arc gas heating apparatus, operating on the high frequency induction principle, and a method of operating the same, involving various novel features of construction and operation, whereby transfer of electrical power from a high frequency energizing coil into the plasma arc is effected at an extremely high efiiciency on the order of 85 percent-90 percent.

In accordance with one novel feature of the invention, a high frequency coil for maintaining a plasma are within the core thereof is constructed of variable rather than constant pitch along its axis, the spacing between adjacent turns being less at each end of the coil than in the middle or median portion of the coil. A preferred coil construction according to the invention consists at each end of a few turns of equal pitch or spacing, and a central portion consisting of a few turns of greater pitch or spacing, but likewise of uniform spacing or pitch between the turns. The strong magnetic end fields of the coil tend to repel the charged particles contained in the plasma arc in a direction toward the central region of the coil interior.

As a result of this construction, upon energizing the coil from a high frequency source while initiating ionization in the core thereof, a plasma core or plasmoid is established within the median portion of the coil core which is roughly of ovoid or tear drop configuration, and extends radiallyoutward from substantially the coil axis to something less than the inner coil diameter, and extends axially of the coil in the opposite directions from the coil midpoint substantially to the point at which the coil pitch changes from the relatively wide spacing in the middle to the relatively narrow spacing at the ends.

This change in the shape ofthe plasma arc from the relatively thin annulus of the conventional construction above described, results as above indicated from the fact that with the coil construction of the present invention, the magnetic field intensity upon high frequency energization is greater at the coil ends than at the middle portion and thus concentrates the plasmoid in the median portion of the coil core in the configuration above described.

In accordance with a further novel feature of the invention, the plasma core produced within the variable pitch, high frequency coil in the manner above described, is greatly enhanced in size and intensified in temperature, by the provision of means for injection through the coil core and over the exterior thereof, of repetitive bursts of charged particles, consisting principally of ions, a portion of which penetrate and are retained within the plasma core to enlarge and intensify the same, while a portion of those directed over the coil exterior are inductively acted upon by the exterior magnetic field of the coil to create additional ions from unionized gas molecules, and to intensify the overall gas heating effect of the coil by the eddy current action of its external magnetic field thereon, whereby the efficiency of power conversion from electrical to thermal is greatly increased as compared to conventional plasma torches.

The means in accordance with the invention for generating and directing these repetitive bursts of ionic particles through and over the high frequency coil consists essentially of a flanged and conically tipped anode electrode and a spaced, annular cathode electrode, mounted coaxially within a tubular enclosure adjacent one end of the coil and coaxial therewith, together with means for establishing a gas flow through perforations in the flanged portion of the anode and thence against the cathode electrode and thence through the aperture in the cathode and through and about the coil, and also means for periodically impressing between the anode and cathode electrodes, unidirectional or direct current pulses of high voltage, applied in polarity such that the cathode is negative with respect to the anode.

These high voltage pulses cause bursts of electrons to be emitted from the cathode, which on passing to the anode, bombard and ionize the gas molecules, which ions owing to the pressure of gas flow and the negative field produced by the flow of electrons between cathode and anode electrodes, are ejected with considerable force in a focused beam through the cathode aperture and thence through and about the high frequency coil. The focusing action results from the conically shaped tip of the anode in relation to the diameter of the cathode aperture, and is adjustable in beam aperture of ejected ions, by adjustment of the spacing between anode and cathode electrodes. The anode-cathode assembly and operation thus functions as an ionic gun in generating an ionic plasma and directing the same through and about the work coil.

Still further in accordance with the invention, the unidirectional, high voltage pulses impressed between the anode and cathode electrodes of the ionic gun are derived from the cyclic charging of capacitance contained in a series resonant load circuit and discharge thereof through a grid controlled gas tube, such as a thyratron, the grid of which is periodically triggered positively with respect to the cathode by conventional means to effect the discharge. The resonant frequency of the load circuit is so adjusted as explained below, that the capacitance discharge current is nonoscillatory and hence unidirectional. The anode and cathode electrodes of the ionic gun are transformer coupled via a two-winding, saturatable core transformer into the capacity discharge circuit, whereby the unidirectional capacity discharge current pulses in traversing the transformer primary winding generate corresponding unidirectional voltage pulses in the secondary winding which are thereby impressed between the anode and cathode electrodes of the ionic gun to actuate the same as aforesaid.

The plasma arc generator of the invention as thus basically constructed and operated, embodies the further novel feature in that it is self starting. The ions produced by the ionic gun and injected thence into the core of the high frequency work coil, provide the requisite source of ions for build up of the are by inductive action thereon of the high frequency magnetic field resulting from energization of the high frequency coil from a high frequency voltage source.

The plasma arc generator as above described, is employed as a gas heater in accordance with the invention by injecting a nonionized gas under pressure into a gas chamber mounted adjacent one end of the high frequency work coil and causing it to flow thence through and over the work coil. The gas is thus heated to an elevated temperature without appreciable ionization thereof, in contrast to conventional plasma arc torches wherein the gas heating action is produced by, as above noted, first dissociating the gas molecules into ions and electrons and thence recombining the same with release of heat.

Having thus described the basic features of the invention in general terms, reference will now be had to the accompanying drawings for a more detailed explanation of the above and other features thereof, wherein:

FIG. 1 is an axial sectional elevation of the basic construction of the high frequency induction type plasma arc gas heater of the invention.

FIGS. 20 and 2b comprise a diagrammatic showing of the gas heater unit and appurtenant electrical circuits for energizing the same, the heater unit being shown in fragmentary enlarged sectional elevation, and FIG. 2b being an extension of FIG. 20 along the line x-x.

FIGS. 2c, 2d, 2e and 2fare graphical plots illustrative of the electrical performance of various components of the apparatus, as mentioned above, and as hereinafter described in detail.

FIG. 3 is an enlarged axial sectional elevation of the anodecathode assembly of the heater unit showing various details of construction for adjusting the anode-to-cathode spacing, for impressing an energizing potential on the anode, and for injecting air under pressure through bores in the anode and thence into the space between the anode and cathode electrodes.

Referring to FIGS. 1 and 2b, the plasma arc gas heater unit of the invention, shown generally at 10, comprises a high frequency induction work coil 11 made of copper tubing. The coil is wound on a core guide 12 of refractory dielectric material, such as quartz. Concentrically disposed about coil 11 are tubular members of progressively increasing diameters and comprising, respectively, a radiator member 13 made of zirconia, molybdenum or the like for purposes explained below, a slab wall, heat insulating member 14, made of a refractory material such as quartz, a heat shielding member 15 also made of a refractory material such as quartz, a metal cooling jacket 16 made preferably of aluminum, and an outer metal shielding member 17, comprising the outer wall of the cooling jacket.

The members l5, l6 and 17 extend between supporting end members 18 and 19, and are mounted therein as shown. The slab wall member 14 is mounted at one end in the end member 18 as shown, while its opposite end is seated in an aperture in the opposite end member 19, and projects therebeyond as shown. The radiator 13 is supported by the slab wall member 14 by means of pins, as at 20, extending radially therebetween. Inlet and outlet connections for the cooling jacket 16 are provided, as at 21, 22, for circulation of a coolant liquid, such as cold water, through the cooling jacket. Referring to FIG. 1, extensions 23a, 24a, of the opposite ends of the work coil 11, extend through inlet and outlet insulating bushings 23, 24, for circulating a coolant liquid, such as cold water through the copper tubing of the work coil.

Mounted on the opposite side of the supporting member 18 from that of the aforesaid coil and tube assembly, is a gas chamber 25. A tubular member 26 extends through the gas chamber and through an aperture in the supporting member 18, said tubular member being outwardly flared thence as at 27 into abutment with the upper end of the radiator 13, as at 28, and into contact with the slab wall member as at 28a, FIG. 2b. Upper portion 260 of member 26 as shown in the drawings, is made of metal, while the lower flared portion 27 is made of dielectric material. The core guide 12 terminates at its upper end in an outwardly flanged terminus 29 which is spaced from the flared portion 27 of the tubular member to provide passageways therebetween as at 30 for purposes explained below.

The gas chamber 25 is provided with an inlet connection 31 for injecting gas under pressure into the chamber from whence it is sprayed into the interior of the tubular member 26 by means of spray nozzles as at 32. 33, mounted on pipe sections. as at 34, 35, which extend through the walls of the tubular member 26 into the gas chamber 25 as shown. Gas under pressure is also supplied to the interior of the tubular member 26 through an auxiliary inlet connection 36, on the lower end of which is mounted a plenum chamber, as at 37, to which is secured a flanged conically tipped anode electrode 38, through which small bores extend from the plenum chamber, as at 39, for injection of the gas from the inlet connection 36 through the bores of the anode electrode and into the space between the anode electrode and a cathode electrode 40. The cathode electrode spans the aperture of the tubular member 26 as shown and has a central aperture coaxial with the tubular member which is spaced from the conically pointed anode electrode, as at 40a. Spaced from the cathode electrode 40 is a perforated metal neutralizer screen electrode 41, which also spans the aperture of the tubular member 26 as shown. The cathode electrode and neutralizer screen are both connected to ground, as at 42. The anode electrode 38 is connected to a conductor 43 which is connected as shown in FIG. 2a through the secondary winding S of a saturatable core, isolating transformer M to ground at 48, said transformer having a primary winding P, connected in the above mentioned thyratron capacity discharge circuit as hereinafter described in detail.

With the gas chamber construction ad arrangement of com ponents described, gas injected into the gas chamber from inlet connection 31 will flow in nonionized state through the spray nozzles 32, 33 and into and through the core guide 12 and work coil 11, and will also flow in part through the passages as at 30 and between the core guide 12 and radiator 13 over the exterior of the work coil. Concurrently therewith gas injected through the auxiliary inlet 36 will in flowing between the anode and cathode electrodes 38, 40, become ionized by the high DC pulsating voltage impressed between these electrodes in the manner generally above described and more in detail below, and will be injected in a focused beam thence in ionized state through and over the work coil 11. Referring to FIG. 2b, the flow of ionized gas from the electrodes 38, 40 is indicated at 49, and that of the combined gases at 5052, Inc.

As shown in both FIGS. 1 and 2b, the helical turns of the work coil 11 are more closely spaced at the opposite ends thereof than at the center of the coil, whereby when the coil is energized with electrical current, the magnetic field intensity or flux density in the core of the coil will be greater at the coil ends than at the center of the coil and will vary in intensity along the core as indicated at 53, 54in the FIG. 2b drawing. This provides a magnetic bottle effect which concentrates the plasma are at the center of the coil as at 55, when produced as described below.

Referring now to FIGS. 2a-2f, Inc., in order to energize the heater unit 10 for plasma arc gas heating, the essential electrical equipment and connections required, include a high frequency oscillator 61, connected across the heater unit work coil 1 1, as at 62. This oscillator applies to the work coil a high frequency voltage of constant amplitude as shown at 63, FIG. 2c.

Referring more particularly to FIG. 2a, the above-mentioned pulser or modulator unit for producing periodic bursts of charged particles between the heater unit cathode and anode electrodes 38, 40, is shown generally at 64. The circuit thereof comprises a grid controlled, triode thyratron tube 65 having connected between its cathode and anode electrodes 66, 67, a series resonant pulse forming network consisting of one or more condensers as at 68, 69, and coils as at 70, 71, connected as shown and comprising an output load circuit for the tube. Transformer M has its primary winding P connected in series with the pulse forming network, and its secondary winding S connected between cathode and anode electrodes 40, 38 of the heater unit 10 via ground connections 42, 48 and connection 43.

A full wave, three phase rectifier unit 73, supplied with three phase power via input leads 74, under control ofPower On and Off buttons 75, 76, supplies direct current voltage to its output terminals 77, 78, the direct voltage source being simulated by DC battery 79 of voltage E in series with its internal resistance 80. The plus terminal 77 of the voltage source is connected over lead 81 of the modulator unit through a diode tube 82 and a charging reactor 83 to the thyratron anode, the cathode of which is connected to the grounded rectifier output terminal 78 as shown.

With the arrangement shown, when the rectifier is switched on, the condensers 68, 69, will charge up to the full DC output voltage E of the rectifier in the manner shown graphically at 84 of the voltage graph FIG. 2d, over the circuit traced from rectifier output terminal 78, through transformer primary P, condensers 68, 69 in parallel and coils 70, 71, charging reactor 83 and diode 82 to rectifier output terminal 77. When the maximum charge is attained, a pulse driver unit 85 actuates a pulse control unit 86 to supply a short positive pulse to the thyratron grid 870, thus to ionize its space path. The charged condensers 68, 69 will thereupon discharge as at 85 of the voltage graph FIG. 2d, through the ionized space path of the thyratron in series with the inductances' 70, 71, condensers 68, 69 and transformer primary winding P. v

If the series inductance, capacity and resistance of the discharge circuit are represented by L, C and R respectively, the resonant frequency of the discharge current will be zero, if

in which case the discharge current will be nonoscillatory and will comprise a single direct current pulse, as shown at 86 of the discharge current graph of FIG. 2e, and the voltage drop across the condensers 68, 69 will be as shown at 85 of FIG. 2d. When the current has dropped substantially to zero, the thyratron will de-ionize and the condensers 68, 69 will again charge up to the voltage E over the charging circuit above traced, and as shown at 87, FIG. 2d. Upon again triggering the thyratron grid positive the condensers will again discharge as at 88, FIG. 2d of the current graph, and so on repetitively. The pulse driver and control units 85, 86, are appropriately adjusted to apply positive pulses to the thyratron control grid to produce the cyclical sequence of operations depicted in FIGS. 2d and 2e.

The condenser discharge current pulses 86, 89, FIG. 2e, in flowing through the primary winding P of the saturatable core transformer M, will produce unidirectional voltage and current pulses in the secondary transformer winding S, as graphically depicted at 90, 91 of FIG. 2f. This results from the fact that as the current pulse 86, FIG. 2e, for example, increases in the transformer primary, it produces a voltage in the transformer secondary which rises almost instantaneously to a maximum value, at which further voltage rise is abruptly cut off by saturation of the transformer coil. When now the current in the transformer primary reaches its maximum value of I, max., FIG. 2e, and decreases thence to zero, no appreciable negative voltage is generated in the transformer secondary, since the transformer core has insufficient time to recover from its saturated state in this time interval. Hence, there is impressed between the cathode and anode electrodes 40, 38 of the heater unit a series of unidirectional voltage pulses, which produce the above described bursts of ionized particles in the gas flow between these electrodes.

Not only is the heater unit rendered self starting by the production of surges of charged particles in the gas chamber as above described, but in addition, these particles are swept through and over the heater coil 11, to increase the overall gas heating effect produced, as compared to conventional constructions. The charged particles from the gas chamber which pass through the heater coil core, add to those produced by high frequency induction therein to intensify the size and temperature of the plasma arc. The heating efficiency is further increased by reaction of the external magnetic field of the work coil 11 on the flow of ionized and nonionized gas from gas chamber 25 and tubular member 26 into the space between the radiator 13 and the core guide 12.

Referring to the enlarged anode assembly view of FIG. 3, the conically pointed anode electrode 38, has an upper portion of cuplike configuration as at a, into which fits a disc member 91a welded thereto, but leaving a space therebetween to provide the aforesaid plenum chamber 37. Said disc plate 910 has centrally located a ball bearing race assembly 92 into which a short nipple section 93 is fastened. An insulator member 94 of ceramic material having secured therein a bushing 95 receives at its lower end, the upper end of nipple 93 in threaded engagement therewith. In the upper end of insulator 94, is secured an internally threaded bushing 97 which receives in threaded engagement the lower end of a long threaded tube 96. lnterposed between the lower end of bushing 95 and a plate 98 secured to the disc member 91a, is a spring washer 99. This spring washer 99 is secured by means of bolts 100a to the disc 91a, which also secures the disc 98 in place. This spring washer exerts pressure between the two elements 98 and 95 to restrict rotation of the disc and anode assembly 91a, 38. A back mounting plate 100 has an internally threaded bushing 101 attached thereto through which tube 96 is threaded. To the upper end of tube 96 is secured by set screw 102 an adjusting member-103. A gas inlet flexible hose 104 is secured to the upper end of tube 96 by clamping ring 105. Through the hose is injected the auxiliary gas supply indicated at 36 of FIGS. 1 and 2b.

In order to raise or lower the anode 38 with respect to the cathode 40, thereby to vary the gap between the two, the adjusting member 103 is rotated clockwise or counterclockwise, as desired, thereby turning with it the threaded tube 96, insulator 94 down to and including the inner race of the ball bearing assembly 92. As a result of this manipulation, the threaded tube 96 threads upwardly or downwardly with respect to the bushing 101 carried by the stationarily mounted backing plate 100, whereby the anode 38 is raised or lowered without, however, rotating the disc member 91a and the anode 38 welded thereto.

It is necessary that the anode and disc assembly 38, 91a remain fixed in a nonrotative position as the anode is raised and lowered with respect to the cathode in order to bring in the electrical connection 43 to the anode. To this end, the input connector 43 to the anode is brought in through a flexible line 106 to a coupler 107 carried by the mounting plate 100 as at 108, the conductor extending thence through an insulator as at 109 and thence to a ringlike flexible connector 110 to which conductor 43 is welded at one point and to an opposite point of which another conductor 111 is welded, the latter extending thence to the disc plate 9/a and being welded thereto. Thus, as the adjusting member 102 is rotated to raise or lower the anode electrode 38 in the manner above described, the ring conductor 110 will correspondingly flex upwardly or downwardly to compensate for the adjustment. The insulator 94 is interposed between the mounting plate 100 and the anode electrode 38 for the reason that the mounting plate is connected to ground.

In a successfully operated embodiment of the invention, the work coil 11 had an outside diameter or CD. of 5 inch. The coil was wound with copper tubing having five-eighths inches OD. The spacing between coil turns at the coil ends was onehalf the tubing OD. and wasequal to the tubing 0D. in the intermediate portion. The coil had eight turns as shown in the drawing and its overall length was 9% inches. The radiator 13 was made of 0.250 inches gauge tubing and had an inner diameter or 1D. of 6 inches. The heat slab wall tube 14 had an ID. of 7% inches The cooling jacket 16 had an ID. of 12% inch and an CD. of I3% inches and was made of aluminum. The annular cathode electrode 40 had an CD. of 7% inches and an ID. of2 inches. The anode electrode 38 had an OD. of 6% inches and the spacing between the electrodes 38, 40, was 0.9 inch. The conical angle 115, FIG. 3, of the anode 38 was about 60 The anode, cathode and neutralizer electrodes 38, 40, 41 were made of aluminum. The distance between the end supports 18, 19 was 18 inches.

The high frequency source 61 operated at a frequency of 4.3105 megacycles and generated a voltage of 3800 volts. The DC output voltage of the rectifier 73 was 8.5 kilovolts.

When the condensers 68, 69, are discharged, DC voltage pulses of 8 kilovolts peak value are delivered to the primary P of output transformer M and hence, since this transformer is unity ratio, DC voltage pulses of 8 kilovolts peak value via transformer secondary S are impressed between the heater electrodes 38, 40. The condensers 68, 69 had capacities of 9.91 microfarad each. The coils 70, 71 had inductances computed in accordance with the formula given above to produce a resonant frequency of. zero in the thyratron discharge circuit.

Referring to FIG. 2d, the interpulse period X between condenser charging and discharging cycles was exactly equal to one-half ofa resonant charging and discharging cycle "Y. The characteristics of the pulse forming network 63-71, Inc, are such that each current pulse on condenser discharge such as 86, FIG. 2e, rises to its maximum value in 0.1 second, has a peak duration of 2 microseconds, and decreases thence substantially to zero in 0.2 microseconds. The peak value I max. of the current pulse is about 75-90 amperes. It has been found experimentally that for maximum conversion of electrical to thermal power, the successive current pulses such as 86 and 89, HO. 2e, should occur at a repetition rate of about 300475 pulses per second. At this repetition rate, the primary winding P of transformer M had an impedance of about 50 ohms and the secondary winding S had an impedance of about 633 ohms.

The plasma arc gas heater of this invention thus differs basically as to construction and mode of operation from previously known types. As above noted, the conventional types operate on the principle of gas ionization into ions and electrons and subsequent recombination thereof for releasing thermal energy. In the high frequency induction coil types, wherein a coil of uniform pitch or spacing is employed for ionizing a flow of gas into electrons and for recombination and release of heat in a plasma are at the inductor output, cessation of the gas flow terminates the action.

in contrast with the high frequency coil construction of this invention employing decreased pitch at the ends as compared to the median portion, gas ionization once initiated is built by the high frequency induction into a sustained plasma are or plasmoid disposed within the median portion of the coil core, as at 55, FIG. 2b. No appreciable gas flow through the coil core is required to sustain the are once established. The external electrical field of the arc is maintained substantially neutral by the dissociated electrons and ions contained therein. Hence, a flow of nonionized gas through the coil core will be heated by the arc with no accompanying substantial ionization of the gas.

However, the are as thus maintained by high frequency energization of the coil alone, tends to be unstable, has relatively little ability to heat gas flowing through the coil core and is relatively inefficient as regards transforming electrical power applied to .the coil into heat.

My experiments have demonstrated, however, that when the high frequency electrical energization of the coil is supplemented by the ion flow from the ion gun, the plasma arc 55, FIG. 2b, is greatly enlarged in volume, increased in temperature and the efficiency of electrical to thermal power conversion greatly increased. These experiments have shown that if the high frequency coil alone is energized, the temperature of the arc is about 3500 C., whereas if the ion gun is also activated, the arc temperature is increased to about 5000 C., and the volume of the are increased by about percent to percent. At the same time, the high frequency current sup plied by the oscillator 61 to the high frequency coil 11, FIG. 2b, is decreased by about 13 percent. In addition, the arc becomes highly stable. The resultant increase in volume and temperature of the arc has thus increased its gas heating ability by at least the product of these increases, i.e., about 85 percent, and the efficiency of power conversion has been correspondingly increased.

As above pointed out, a portion of the ions from the ion gun, penetrate and intensify the plasma arc, while a portion of those directed over the coil exterior are inductively acted upon by the exterior magnetic field of the coil to dissociate additional nonionized gas molecules into ions and electrons, thereby further to intensify the gas heating action. This heating action is still further intensified by the radiator 13 which is preferably made of zirconia. Although this material is substantially an electrical insulator at ambient temperatures, it becomes electrically conductive when heated to temperature it attains during operation of the plasma arc. At this temperature, ions are given off by the radiator, both thermally and by inductive action of the external magnetic field of the high frequency coil. Some of the ions thus produced also penetrate the plasma are further to intensify the same. In addition, the radiator acts as a heat reflector to confine the heating action therein.

In operation, the plasma arc gas heater of the invention is capable of heating large amounts of flowing gas to temperatures ranging from about 2000--6000 F. at operating efficiencies approaching percent.

The invention finds utility, among other applications, in the vapor phase production of titanium pigments for preheating the oxygen gas required for reaction with titanium tetrachloride to temperatures such as to effect such reaction and thereby produce a titanium dioxide pigment of excellent characteristics.

What I claim is:

1. Plasma arc gas heatingapparatuscomprising in combination: a helically'woiiiid high frequency induction coil, means coaxially mounting said coil in a tubular enclosure open at one end for exit of heated gas and closed at the opposite end, a pair of spaced electrodes mounted in said closed end of said tubular enclosure, means for injecting a flow of gas therein between said electrodes and thence through and over said coil, additional means for injecting gas into said tubular enclosure at a point between said electrodes and said coil, and elec trical connections to said electrodes for impressing a gas ionizing voltage therebetween.

2. Apparatus according to claim 1, wherein said pair of electrodes comprise a conically tipped anode electrode mounted coaxially with said coil and axially spaced therefrom, and an annular cathode electrode disposed between said anode and said coil and mounted coaxially therewith, said cathode spanning said tubular enclosure, and the conical tip of said anode facing said cathode.

3. Apparatus according to claim 2, wherein said anode electrode is provided with passageways extending therethrough and means are provided for injecting gas through said passageways into the space between said electrodes.

4. Apparatus according to claim 1, wherein successive turns of said coil are more closely spaced at the ends thereof than in the portion intermediate said ends, for intensifying the magnetic field produced at said coil ends as compared to said intermediate portion when said coil is traversed by electrical current and for thereby concentrating a plasma are within said intermediate portion of said coil.

5. Apparatus according to claim 4, wherein successive coil turns are equispaced at each of said coil ends and are equispaced throughout said intermediate portion but at greater spacing than at said coil ends.

6. Apparatus according to claim 1, wherein said coil is a helically wound metal tube, and wherein means are provided for circulating a coolant fluid through said tube.

7. Apparatus according to claim 1, wherein voltage energizing means are connected between said spaced electrodes for periodically impressing therebetween unidirectional voltage pulses of magnitude to ionize gas flow in the gap between said electrodes for discharge thence of ionized particles through and about said coil, and wherein a source of sustained high frequency voltage is connected between said coil ends whereby a plasma arc is initiated and sustained within said coil.

8. Apparatus according to claim 7, wherein said voltage energizing means comprises a grid controlled thyratron tube having a series resonant inductance and capacity discharge circuit bridged between its cathode and anode electrodes, and having connected in parallel therewith a charging circuit including a direct current voltage source and a charging impedance in series, for charging said capacity to the voltage of said source, pulsing means for periodically firing said thyratron to discharge said capacity and produce unidirectional current pulses in said resonant discharge cir- 

1. Plasma arc gas heating apparatus comprising in combination: a helically wound high frequency induction coil, means coaxially mounting said coil in a tubular enclosure open at one end for exit of heated gas and closed at the opposite end, a pair of spaced electrodes mounted in said closed end of said tubular enclosure, means for injecting a flow of gas therein between said electrodes and thence through and over said coil, additional means for injecting gas into said tubular enclosure at a point between said electrodes and said coil, and electrical connections to said electrodes for impressing a gas ionizing voltage therebetween.
 2. Apparatus according to claim 1, wherein said pair of electrodes comprise a conically tipped anode electrode mounted coaxially with said coil and axially spaced therefrom, and an annular cathode electrode disposed between said anode and said coil and mounted coaxially therewith, said cathode spanning said tubular enclosure, and the conical tip of said anode facing said cathode.
 3. Apparatus according to claim 2, wherein said anode electrode is provided with passageways extending therethrough and means are provided for injecting gas through said passageways into the space between said electrodes.
 4. Apparatus according to claim 1, wherein successive turns of said coil are more closely spaced at the ends thereof than in the portion intermediate said ends, for intensifying the magnetic field produced at said coil ends as compared to said intermediate portion when said coil is traversed by electrical current and for thereby concentrating a plasma arc within said intermediate portion of said coil.
 5. Apparatus according to claim 4, wherein successive coil turns are equispaced at each of said coil ends and are equispaced throughout said intermediate portion but at greater spacing than at said coil ends.
 6. Apparatus according to claim 1, wherein said coil is a helically wound metal tube, and wherein means are provided for circulating a coolant fluid through said tube.
 7. Apparatus according to claim 1, wherein voltage energizing means are connected between said spaced electrodes for periodically impressing therebetween unidirectional voltage pulses of magnitude to ionize gas flow in the gap between said electrodes for discharge thence of ionized particles through and about said coil, and wherein a source of sustained high frequency voltage is connected between said coil ends whereby a plasma arc is initiated and sustained within said coil.
 8. Apparatus according to claim 7, wherein said voltage energizing means comprises a grid controlled thyratron tube having a series resonant inductance and capacity discharge circuit bridged between its cathode and anode electrodes, and having connected in parallel therewith a charging circuit including a direct current voltage source and a charging impedance in series, for charging said capacity to the voltage of said source, pulsing means for periodically firing said thyratron to discharge said capacity and produce unidirectional current pulses in said resonant discharge circuit, and means electrically coupling said spaced electrodes disposed in said tubular enclosure, respectively, to points of potential difference in saId resonant circuit.
 9. Apparatus according to claim 8, wherein said spaced electrodes disposed in said tubular enclosure are transformer coupled to said resonant discharge circuit through a two-winding saturatable core transformer. 